PYROMETER.
THE name Pyrometer, from mvp, fire, and pt,Erpoy, a mea sure, is given to those instruments which measure the ex pansion of solid bodies by heat, and to another class of in struments which measure degrees of heat above those which can be indicated by the mercurial thermometer. This last application of the name is by no means judi cious, and consequently pyrometers of this kind should be considered as thermometers. We shall, however, proceed to describe these instruments in the order of their inven tion, and under the two different classes now mentioned.
Tun machine, which we shall describe nearly in the words of the original inventor, is represented in Plate CCCCLXXI. No. II. Fig. 1, where AAA is a piece of iron turned up perpendicularly at one end, the return be ing inch high. The other end, distant from it a inch, is also turned up, and is turned back again, so as to make a broad square plate, the side of which is 2 inches. The iron itself is 1 inch wide and thick.
Upon the iron plate stands a brass machine, which is drawn by itself in Fig. 2, where it is represented larger, and seen from another side, the better to discover its parts, which are marked with the same letters as in Fig. I. This is fixed to the iron by two screws X, X, which are its legs. D is a circular plate of inches diameter, divided into 300 equal parts. This divided plate stands upon four equal pillars E, E, E. E, which join it to the lower brass plate ; between these two plates there is a perpendicular steel axis F, which has on its lower part a pinion of 6 leaves, and on its upper a wheel of 60 teeth, marked G : there is also another axis I H, supported by a cock from the upper plate, and which axis receives the index I K ; having at its lower end a pinion of 6 leaves to take the teeth of the wheel G. The index, by one turn of the pinion H, is car ried round to all the divisions. There is, besides, a little rack L with teeth, which take the leaves of the pinion F, while the rack slides along under two small cocks P, P, where it is pressed towards the pinion F by two small screws M M, or drawn from it, as there is occasion, that the teeth may neither stick nor be loose. The teeth of
the rack are 25 in an inch, and as it moves forward and backward, the pinion F is carried round, and consequently the wheel G, which carries round the pinion II, together with the index I K. Let the rack have run one inch; then F and G will have turned round 4 times and 4; and conse quently the pinion H will have gone round 10 x 46, that is, 411 times. Hence 414 x by 300 = 12,500 parts. There fore one division corresponds to the 12,500th part of an inch.
Fig. 3. represents a square bar,or parallelopiped of metal, upon which the experiment is made, 5 inches long, and of an inch thick. One end of it 0 has a small tail, that it may communicate no sensible degree of heat to the iron A A ; it is received in a notch near B, and fixed by the rew C. Its other end N has a hole in it, through which goes the screw Q, which makes it fast to the rack L.
The bar, being thus fixed, cannot become longer, with out pushing forward the rack L, and moving round the hand I K; so likewise, when it becomes shorter, it must move the contrary way. The weight of the bar is support ed by a piece of watch spring between the square iron and brass plates E A. To apply conveniently the flame of spi rits, there is a box of brass, 34 inches long, inch wide, and inch deep, covered at top with a piece of blue stone, such as will bear the fire. It has a long hole cut through the middle, into which is let in a brass plate T, which has 5 small equidistant holes fl parts of an inch asunder, and inch in diameter, to transmit the wicks of the lamp. This lamp has 4 feet, which closely take in the iron A be tween them, that the flame may come against the middle of the bar; but the bottom of the lamp must not touch the iron. The distance between the bottom of the bar and the upper surface of the lamp must be half an inch, that the cotton wicks, which stand up flo- of an inch, may give an equal flame. The cotton threads must be fine and even, and 5 of them must make a wick of about of an inch. If the wick is drawn up too high the flame will be too large. In experiments with highly rectified spirits of wine, there was always an equal quantity put into the lamp.